In general, the present network device manufacturers usually integrate an antenna diversity function into a wireless network device such as a wireless access point or a wireless router to provide the best signal transmitting and receiving quality to the wireless network device and achieve a full coverage of transmitting and receiving signals without worrying about dead spots. The present wireless network devices further includes another antenna inside a casing, in addition to the antenna exposed from the casing, and most of the internal antennas are chip antennas, printed monopole antennas and inverted-F antennas.
However, the wireless network devices tend to be developed with a compact design, so that when wireless network device manufacturers design a wireless network device, the space of a circuit board in the wireless network device for installing another antenna becomes increasingly smaller. Furthermore, the requirement for various different types of components and the mechanical design of the wireless network device further cause more limitations to the space of designing and installing another antenna, so that the other antenna is unable to achieve the best signal transmitting and receiving quality, and the reasons are elaborated as follows:    1. Chip antenna 10: Referring to FIG. 1, a low temperature co-fired ceramic (LTCC) technology is used for producing a chip antenna 10 featuring a small size, and thus the LTCC technology obviously provides a flexible way of installing the chip antenna 10 in a limited internal space of the wireless network device. However, the internal mechanism of the wireless network device is usually different from the coefficient of a circuit board 2 for the practical applications of the chip antenna 10, and thus the chip antenna 10 often cannot be installed according to the best condition as recommended and described in a data sheet of the chip antenna 10, and the bandwidth performance and the radioactive efficiency become very low, and the additional cost of installing a chip antenna cannot meet the requirement of a low cost.    2. Printed monopole antenna 12: Referring to FIG. 2, a printed monopole antenna 12 is printed on a circuit board 2 through a microstrip, and a portion of the microstrip is a feed line 122, and a signal line 120 is extended from the feed line 122 across a grounding plane 20 of the circuit board 2. The advantage of the printed monopole antenna 12 resides on its simple architecture and easy design, and any shape of printed monopole antenna 12 can be formed to fit the layout of a circuit board 2. However, different shapes of the printed monopole antennas 12 for wireless network devices are designed according to the designer's habit and the coefficients of mechanism and circuit board 2 usually come with a big discrepancy and a low bandwidth performance, and thus causing tremendous problems to antenna manufacturers and designers.    3. Inverted-F antenna 14: Referring to FIG. 3, the inverted-F antenna 14 is printed on a circuit board 2, and the circuit board 2 has a grounding plane 20 disposed proximate to an edge of the circuit board 2, and the inverted-F antenna 14 is comprised of a feed line 140, a short circuit line 142 and an antenna body 144, wherein an end of the feed line 140 and an end of the short circuit line 142 are connected to the grounding plane 20, and another end of the short circuit line 142 is extended towards an edge of the circuit board 2, and an end of the antenna body 144 is connected to the short circuit line 142, and another end of the antenna body 144 is extended along an edge of the circuit board 2 and connected to another end of the feed line 140 and then further extended along the edge of the circuit board 2 to a predetermined length. From the description above, the inverted-F antenna 14 is a change of the shape of a printed monopole antenna 12. Unlike the shape of the printed monopole antenna 12, the special shape of inverted-F antenna 14 allows the inverted-F antenna to occupy a smaller area of the circuit board 2 than a printed monopole antenna 12. Referring to FIG. 4, the widths of the feed line 140 and the short circuit line 142 are both 0.85 mm, their lengths are 5 mm, the distance between the feed line 140 and the short circuit line 142 is 2.3, the width of the antenna body 144 is 0.7 mm, and the length of the antenna body 144 is 18.63 mm. Referring to FIG. 5, the testing and measurement of a circuit design indicate that the bandwidth of a return loss −10 dB falls within 2.338˜2.508 GHz, and the inverted-F antenna 14 generally comes with a problem of insufficient bandwidth.
In view of the description above, a common drawback of the aforementioned antennas 10, 12, 14 resides on low bandwidth and poor couple error vector magnitude (EVM), and thus affecting the transmission throughput. Since the wireless network device tends to be small in size, and the mechanical requirement tends to be high, the position for installing an antenna is very limited, and thus it is necessary to develop a wideband antenna with a simple shape and a small size.